Cold sores are caused by HSV1
Catching a cold sore puts you at risk of Alzheimer's disease, mounting evidence suggests.
The herpes virus behind cold sores is a major cause of the protein plaques that accumulate in the brains of people with Alzheimer's, scientists have shown.
On the plus side, the latest discovery by the University of Manchester team may mean antiviral drugs used to treat cold sores could also prevent dementia.
The findings are published in the Journal of Pathology.
Professor Ruth Itzhaki and colleagues found DNA evidence of the herpes simplex virus (HSV) type 1 in 90% of plaques in Alzheimer's disease patients' brains.
They had previously shown that HSV1 infection of nerve-type cells in mice leads to deposition of the main component of the plaques - beta amyloid. And that the virus is present in the brains of many elderly people and that in those people with a specific genetic factor, there is a high risk of developing Alzheimer's disease.
Taken together, the researchers say the findings strongly implicate the cold sore-causing virus as a root cause of Alzheimer's dementia.
Professor Itzhaki said: "We suggest that HSV1 enters the brain in the elderly as their immune systems decline and then establishes a dormant infection from which it is repeatedly activated by events such as stress, immunosuppression, and various infections."
In turn, this causes damages the brain cells, which die and then disintegrate, releasing the proteins which develop into amyloid plaques, she said.
The researchers now plan to test whether antiviral drugs used to treat cold sores, which block the action of HSV1, might stop the cell damage that leads to Alzheimer's.
Another possibility is vaccination against the virus to prevent the development of the disease in the first place.
Rebecca Wood of the Alzheimer's Research Trust said: "This could lead to new treatments for Alzheimer's, based on existing antiviral agents. However, the underlying causes of Alzheimer's disease are still unknown. Much more research is needed if we are to offer hope to the 700,000 people in the UK who live with Alzheimer's and related dementias."
Professor Clive Ballard of the Alzheimer's Society said: "Although the new research provides some additional evidence supporting a link between the herpes virus and Alzheimer's disease there is still uncertainty around whether this is a promising avenue of research."
Most people become infected with HSV1, which then remains dormant in the facial nerve emerging periodically in 20-40% of those infected to cause cold sores.